This invention relates to a low-pass filter used with a high-frequency transmission circuit of a digital portable telephone.
As shown in FIG. 3, in a high-frequency transmission circuit of a digital portable telephone, a voice modulation signal 20 is mixed with a high-frequency signal from a voltage-controlled oscillator 21 by a mixer 22, the mixed signal is amplified by a power amplifier 23, the output signal level of the power amplifier 23, namely, a signal responsive to transmission power (about -20 dB) is fed back into a gain control terminal of the power amplifier 23 by a transmission power detection coupler 24 for controlling the power amplifier 23 so as to maintain the transmission power at a predetermined level, and a harmonic signal occurring in the power amplifier 23 is removed through a low-pass filter 25. Then, the resultant signal is transmitted from an antenna 27 through a transmission/reception switch circuit 26 which switches between the transmission circuit and a reception circuit 28.
As shown in a circuit diagram of FIG. 4A, the conventional low-pass filter 25 comprises a parallel circuit of a coil L1 and a capacitor C1 inserted between an input terminal P1 and an output terminal P2, a capacitor C2 inserted between the input terminal P1 and ground, and a capacitor C3 inserted between the output terminal P2 and ground. As shown in a perspective view of FIG. 4B, the input terminal P1, the output terminal P2, and ground terminals G are placed on the side faces of a laminate chip 30 containing the coil L1 and the capacitors C1-C3.
FIG. 4C shows the lamination structure of the low-pass filter. The low-pass filter is manufactured by a print method or a sheet method. To use the sheet method, a dielectric layer 1a on which a ground electrode 2a is printed, a dielectric layer 1b on which capacitor electrodes 3a and 3b forming the capacitors C2 and C3 together with the ground electrode 2a are printed, a dielectric layer 1c on which a capacitor electrode 3c forming the capacitor C1 together with the capacitor electrodes 3a and 3b is printed, a plurality of dielectric layers 1d forming a gap layer, a dielectric layer 1a on which a coil 4a forming a part of the coil L1 in printed, a dielectric layer if on which a coil 4b of the remaining part of the coil L1 is printed, the dielectric layer 1f being formed with a through hole 5a for connecting an and part of the coil 4b to the coil 4a, a plurality of dielectric layers 1g forming a gap layer, a dielectric layer 1h on which a ground electrode 2b is printed, and a dielectric layer 1i formed with a mark 6a and forming a surface protection layer are laminated. The laminate body is cut for each chip, then the chips are fired and the terminals P1, P2, and G are attached to the side faces of each chip, manufacturing products.
As shown in FIG. 5A, the transmission power detection coupler 24 comprises a coil L2 connected to an output line via an input terminal P3 and an output terminal P4 and a coil L3 magnetically coupled to the coil L2 and connected at both ends to detection signal output terminals P5 and P6. As shown in a perspective view of FIG. 5B, the input terminal P3, the output terminal P4, the detection signal output terminals P5 and P6, and ground terminals G are placed on the side faces of a laminate chip 31 containing the coils L2 and L3.
FIG. 5C shows the lamination structure of the coupler 24. A dielectric layer 1j on which a ground electrode 2c is printed, a plurality of dielectric layers 1k forming a gap layer, a dielectric layer 1m on which a leader electrode 4c of the coil L2 is printed, a dielectric layer 1n on which a coil 4d as the main part of the coil L2 is printed, the dielectric layer 1n being formed with a through hole 5b for connecting an end part of the coil 4d to the leader electrode 4c, a dielectric layer 1p on which a coil 4e as the main part of the coil L3 is printed, a dielectric layer 1q on which a leader electrode 4f of the coil L3 is printed, the dielectric layer 1q being formed with a through hole 5c for connecting an and part of the leader electrode 4f to the coil 4e, a plurality of dielectric layers 1r forming a gap layer, a dielectric layer 1s an which a ground electrode 2d is printed, and a dielectric layer 1t formed with a mark 6b and forming a surface protection layer are laminated. The laminate body is cut for each chip, then the chips are fired and the terminals P3-P6 and G are attached to the side faces of each chip, manufacturing products.
An described above, in the high-frequency transmission circuit of the conventional digital portable telephone, the low-pass filter 25 and the coupler 24 are formed separately, thus inhibiting cost reduction accomplished by decreasing the number of mounted part and also miniaturization accomplished by decreasing the mount area in circuit design.